15 000 ellipsoidal binary candidates in TESS: Orbital periods, binary fraction, and tertiary companions

Abstract

We present a homogeneously-selected sample of 15 779 candidate binary systems with main sequence primary stars and orbital periods shorter than 5 days. The targets were selected from TESS full-frame image lightcurves on the basis of their tidally-induced ellipsoidal modulation. Spectroscopic follow-up suggests a sample purity of 83 ± 13 per cent. Injection-recovery tests allow us to estimate our overall completeness as 28 ± 3 per cent with Porb <3 days and to quantify our selection effects. 39 ± 4 per cent of our sample are contact binary systems, and we disentangle the period distributions of the contact and detached binaries. We derive the orbital period distribution of the main sequence binary population at short orbital periods, finding a distribution continuous with the log-normal distribution previously found for solar-type stars at longer periods, but with a significant steepening at Porb ≲ 3 days, and a pile-up of contact binaries at Porb ≈0.4 days. Companions in the period range 1–5 days are an order of magnitude more frequent around stars hotter than ≈6250 K (the Kraft break) when compared to cooler stars, suggesting that magnetic braking shortens the lifetime of cooler binary systems. However, the period distribution in the range 1–10 days is independent of temperature. We detect resolved tertiary companions to 9.0 ± 0.2 per cent of our binaries with a median separation of 3200 AU. The frequency of tertiary companions rises to 29 ± 5 per cent among the systems with the shortest ellipsoidal periods. This large binary sample with quantified selection effects will be a powerful resource for future studies of detached and contact binary systems with Porb<5 days.

Dynamical Masses for the Hyades Binary System vB 120

“…Sector 32 clearly shows a single dominant period of about 8.5 days, consistent with the estimate of Douglas et al (2019). Sector 5, the one on which Green et al (2023) based their result, as well as Sectors 43 and 44, display a more complicated structure. Given the similarity of the properties of the two components, including their brightnesses, it seems plausible that both stars contribute to the variability with similar periods of roughly 8.5 days.…”

“…It is also close to the v i sin estimate of Griffin (2012) for the primary (4.6 ± 0.7 km s −1 ), although his estimate for the secondary is much lower (2.1 ± 0.9 km s −1 ). A shorter rotation period of 4.41 days (Green et al 2023) would require Fuhrmeister et al (2022) predicted a rotation period of 9.2 days using a statistical relation by Mittag et al (2018), which is rather close to the period seen in the TESS data.…”

“…In that case, it is possible that the corresponding peaks and troughs in the light curve happened to line up for the two stars in Sector 32 (or the spots on one star disappeared), but were out of phase in the other sectors. We speculate that this misalignment could have led to the shorter period reported by Green et al (2023) from Sector 5. Alternatively, perhaps only one star is spotted, and its surface features come and go, or has two spots on opposite sides, one of which comes and goes.…”

“…Douglas et al (2019) used data from the K2 mission, and inferred a photometric period of 8.61 days, presumably due to rotation. Green et al (2023) examined the TESS observations, and reported a period of 4.41 days, about half as long.…”

Discovery of a Proto–White Dwarf with a Massive Unseen Companion

“…The sampling of the RVs from spectroscopy is insufficient for deriving the period, and therefore we use the lightcurve data to measure the period. Stars in close binary systems show periodic changes in flux due to eclipses, heating of the companion due to the primary (reflection binary; Vaz 1985;Schaffenroth et al 2022), or distortions of their surfaces induced by the tidal forces of the companion (ellipsoidal modulation; Kopal 1959;Green et al 2023). We combine the data from both the g and r filters of ZTF and normalize the data to capture only the fractional variations about the median flux.…”